1. Field of the Invention
The invention relates to a novel squeegee assembly used in a screen printing apparatus, and in particular to a squeegee assembly with a positioning mechanism for adjusting the position of the squeegee blade within the assembly such that the length of the squeegee blade extension remains constant during extended use of the blade in a screen printing process.
2. Summary of Related Art
Screen printing is an important process in making automotive glass and other glass components, for making printed circuit boards, and for a number of other applications where a specific pattern is printed on a hard substrate surface. In the glass manufacturing process, screen printing is used to print the electrically conducting networks for heater arrays on rear windows and back lights and to print shaded bands around the edges of the window glass. In printed circuit boards, screen printing is used to print the conducting networks on the circuit board.
Screen printing presses are also used for body printing-package printing applications. Presses print on both flat and curved surfaces and are used in a variety of industries.
In the various manual and automatic screen printing apparatus, a flood bar and a squeegee blade are mounted on a carriage in the printing press. The press includes a control system to control both vertical and horizontal motion of the squeegee blade assembly and/or the carriage. A screen with the desired pattern is placed in contact with the object being printed. The flood bar is used to distribute ink across the screen in a first pass over the screen. The flood bar is raised and the squeegee blade is lowered and drawn across the screen to force the ink through the screen onto the workpiece to be printed upon in a pattern determined by art work formed on the screen.
In some presses, the drive carriage will include only a single squeegee blade. The squeegee blade is pressed across the screen in one direction to force ink through the screen in any of the apertures in the screen coating. A return run of the squeegee blade back across the screen forces the ink onto the surface of the workpiece.
The squeegee blade is made of rubber and is typically mounted in a holder consisting of two face plates extending over the entire length of the squeegee rubber, clamping the squeegee rubber therebetween. In order to achieve pressure on the working edges of the squeegee on a screen, without ripping the screen, the elasticity of the rubber material in the squeegee blades is utilized. This takes place in such manner that the clamping location of the squeegee blade between the face plates is provided at a preferred distance from the working edge of the squeegee blade. The elastic rubber material of the squeegee blade extending from the face plates develops an even pressure over the entire length of the working edge during the printing process.
One of the disadvantages of the squeegee assemblies is that the working edge of the squeegee blade deteriorates after only a relatively short period of use, so that the squeegee rubber is no longer in the position to produce the contact pressure necessary for a satisfactory printing result. In addition, the working edge of the squeegee blade wears unevenly such that the contact pressure is uneven and the quality of the print deviates from acceptable standards.
The positioning and physical condition of the squeegee blade effects both the quality of the printing and the life of the screen used in the printing process. If the squeegee blade is not properly mounted in the holder, waves or buckles will occur in the blade as pressure is applied and the force used to print the pattern will be uneven. Consequently, the printed pattern will not be uniform.
The stiffness or resiliency of the squeegee blade is an important factor in the printing process. The resulting pattern printed on a substrate may be effected by changes in the resiliency of the blade material.
Nicks or chips in the edge of the squeegee blade will also cause printing defects. In addition, the rough edges have a tendency to catch on and rip a screen. Once a screen is ripped, it is no longer usable and a new screen must be installed. The time and costs involved with unscheduled screen changes have an adverse effect on the overall screen printing process.
In order to overcome the disadvantages noted above, the squeegee blade will be reconditioned or replaced on a frequent basis. Because the cost of the squeegee blades is not insignificant, most users will attempt to recondition the squeegee blades to increase the life of the blade. One of the desired features is the ability to take one holder from the carriage and place a second holder in the carriage without having to adjust the carriage drives and positioning systems. In most cases, the carriage must be adjusted each time the squeegee blade is changed. Such carrier adjustment requirement results in additional manpower requirements, production delays, and scrap material.
When the blade is reconditioned, the working edge is trimmed, ground, and/or sanded to provide a new, smooth edge which produces the desired contact pressure.
One of the major problems in reconditioning the squeegee blade is providing a squeegee assembly which will enable a used squeegee blade to be attached to the holder in a manner which will impart to the blade the same rigidity or stiffness as that possessed by the blade prior to being reconditioned. This requires a blade which after being reconditioned one or more times is still able to extend beyond the squeegee holder the same distance as a new blade.
A desired feature for the squeegee blade holder is the ability to maintain the same blade stiffness by allowing the working edge of the blade extending from the holder to be maintained at a constant distance. The squeegee blade exhibits a degree of stiffness or rigidity which is essentially independent of the height dimension of the blade and solely dependent on the size of that part of the blade which extends beyond the squeegee holder. The holder should provide a convenient and simple means of retaining the blade in the squeegee holder and for adjusting the extension of the blade from the holder.
In the screen printing industry, there is still a need for an improved squeegee holder with a means for retaining the squeegee, and for adjustably positioning the squeegee in the holder to maintain the specified blade extension and resulting pressure on the working edge of the blade. An improved squeegee holder with total adjustability of the extension distance of the squeegee blade from the holder will decrease the blade material which must be scrapped, and decrease the amount of time needed to change out and adjust a squeegee holder.
A number of U.S. patents have discussed a squeegee holder assembly for use with carriers on various the printing systems. U.S. Pat. No. 5,078,063 to Johansen et al. discloses a squeegee holder with a levelness control and a pressure control for the squeegee blade. A wedge is positioned between a stationary body and the squeegee blade. A micrometer is used to adjust the level of the squeegee.
An alternative squeegee arrangement is shown in U.S. Pat. No. 4,989,512 to Lindstrom et al. The squeegee is maintained at an angle and includes a plurality of defined support positions. The squeegee holder has two plates to secure the squeegee, and both of the plates have three grooves into which a flat bar is inserted. The three sets of grooves are at different heights on the face plate and the position of the squeegee blade in between the plates is determined by the positioning of the flat bar. The squeegee blade can be adjusted only in the incremental positions based on the spacing of the grooves.
U.S. Pat. No. 4,841,854 to Bubley shows a squeegee assembly with two face plates and a center bar. The face plates and the center bar have ribbed construction such that the position of the center bar determines the extension of the squeegee blade from between the plates. In order to adjust the position of the blade, all of the retention screws used to clamp the two face plates to secure the squeegee must be loosened. Retaining the squeegee blade in an even alignment is very difficult because there is no clamping means to retain the blade when an adjustment is taking place.
U.S. Pat. No. 4,779,529 to Ericsson discloses a squeegee system which maintains the squeegee blade at an obtuse or an acute angle, irrespective of the direction of relative squeegee movement.
A plurality of independent squeegee holder members are shown in U.S. Pat. No. 4,638,733 to Schneider et al. The squeegee holder has holder members which are guided for mutually independent reciprocating movement in the contact direction, and are held under contact force. Because the holder members are independent, the squeegee blade can better follow any unevenness in the screen during the printing process. This holder is used primarily when different wall thicknesses are encountered during the printing process.